Butterfly lights the way to better thermal imaging

Studying the properties of iridescent butterfly wings could help engineers develop temperature sensors that are smaller and faster, according to researchers in the US. They also say that the technology could work without the need for expensive and cumbersome cooling technology and could also have implications for imaging technology such as thermal night vision and medical diagnostics.

There are many ways to detect heat, such as measuring the change in electrical resistance when certain substances change temperature. But to remain sensitive to incoming radiation, these devices must be constantly cooled, otherwise they would continue to register the presence of a heat source for some time after it has been moved away. As a result, the most sensitive thermal imagers require liquid-helium refrigeration. Since the heat sinks required are relatively large and power-hungry, this limits the minimum size and efficiency of the sensors. These requirements pose severe challenges for those designing portable equipment, such as thermal-imaging goggles. Indeed, goggles pose a particular problem because an ideal pair would be transparent to visible light, which is difficult to achieve with heat sinks in the way.

Now, Radislav Potyrailo and colleagues at the General Electric Global Research Centre and the University of Albany in New York have shown that the goals of high sensitivity and convenience could both be satisfied with a little help from nature. They have created a material inspired by the wings of the Morpho butterfly, which are covered with scales that reflect light at some wavelengths and absorb it at others. While this process is not completely understood by scientists, it is known that as the wing heats up, the intensity of the different wavelengths of visible radiation reflected changes slightly, which alters the colour of the wing.

Easy to chill

Potyrailo's team decided to investigate this effect to find out whether the same principle could be used to construct a synthetic temperature sensor. If it could, it would have a significant advantage over current temperature sensors, because the wings of Morpho butterflies are made of chitin, which is a natural polymer with a much lower heat capacity than the metals and semiconductors used in today's sensors. This means that a sensor using this technology could cool down quickly without heat sinks.

Potyrailo's team tested the infrared absorption of Morpho scales and found that when the scales were heated from one side by infrared radiation, thermal expansion caused the ridges to move slightly further apart, thus changing the wavelengths reflected and absorbed when white light hit the scales from the other side. The effect was accompanied by a slight reduction in the refractive index of the chitin.

Decorating with nanotubes

Building on previous work by other researchers that revealed that decorating a material surface with carbon nanotubes enhances its ability to absorb infrared radiation, the team showed that the wings absorbed infrared better if carbon nanotubes were added to the exposed surface. As a bonus, because carbon nanotubes have excellent thermal conductivity, the decoration helped to diffuse heat through the chitin away from the site of irradiation, thus providing a molecular heat sink.

The research is still at an early stage, and the researchers will need to find a way to produce the nanostructured chitin – or a similar material – synthetically before they can produce a viable sensor. But the researchers believe the work could one day lead to relatively cheap, multicolour thermal-image sensors, in which mid-infrared would appear as one colour and far-infrared as another. "If we have a very small pixel size, then in the same or neighbouring pixels we can create these 'Christmas trees' that are responding to different regions of the infrared spectrum and they will be responding with different colours. These days it's either several chips that are combined together or very sophisticated, very complicated design of heat sensor," says Potyrailo.

Clever use of natural structures

"I find the [work] to be a very clever use of the natural photonic structures found on the wing scales of tropical Morpho butterflies," says Mohan Srinivasarao of Georgia Institute of Technology, an expert on butterfly wings. "In my opinion, the more we look carefully at these structures, the more we are going to find more ways of using them; and, of course, the butterflies may not have created these structures for the uses we may find as we look more closely."

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2 comments

Morphos scales-resonators

A temperature sensor converts the infrared radiation into visible light. The resonant cavities of the Morphos-butterfly scales do the same thing. Moreover, doping these scales with the carbon single-walled nanotubes increases the infrared radiation absorption, thus, increasing the coversion efficiency. This work brings out beautifully the nature's way of doing things in the simplest and the lightest way possible. And, here, you try to improve on it! However, what would happen, if one "dopes" these scales with just a layer of graphene?

Emphasis

Technically and strictly speaking I agree with you that: "But to remain sensitive to incoming radiation, these devices must be constantly cooled, otherwise they would continue to register the presence of a heat source for some time after it has been moved away. As a result, the most sensitive thermal imagers require liquid-helium refrigeration." However, in practice microbolometers arrays, which have been used in many IR cameras for decades now, work pretty well and without overt cooling, while the cryo cooled units typically require liquid nitrogen as opposed to liquid He. No doubt this current research will be of great practical value but I think the way you have described current technology does not reflect the reality.